We propose that the development of the secretory IgA (sIgA) system of mammary tissue is regulated by factors other than direct antigenic stimulation of this tissue and that it is dependent on Gut Associated Lymphoid Tissue (GALT) for a source of IgA precursor cells committed to synthesis of antibodies (Ab) directed against intestinal antigens. Thus far we have determined that development of the IgA-containing cells in rabbit mammary tissue begins relatively early in pregnancy and that the rate of development appears to correlate with the rate of development of the glandular tissues of the mammary gland. Our major objectives now are: a) to determine appropriate routes of immunization to elicit a major response of the IgA system of GALT and mammary tissue, and b) to determine if IgA precursor cells derived from GALT home to developing and/or lactating mammary tissue, and if expansion of the IgA cell system in mammary tissue can occur in the absence of an external source of precursor cells. For the immunization studies, various routes and schedules will be employed and responses will be followed by means of PFC, RFC and immunofluorescent assays of lymphoid cell preparations from various tissues. In vitro culture of the lymphoid cell preparations will be done to allow maturation of in vivo stimulated cells in order to track the development of IgA precursor cells in various tissues. In order to determine the source(s) of IgA cells in mammary tissue, cell transfer studies will be done with recipients at various stages of pregnancy and with donor cells of various tissue origin and at various stages of maturational sequence. Additionally, explants of mammary tissue from pregnant rabbits will be cultured in media containing various hormones and subsequently examined to determine if expansion of the IgA cell system can continue in the absence of an external source of precursor cells. Further studies relate to identification of factors regulating homing and/or development of IgA cells in mammary tissue.